Partial coherence mitigation in video measurement systems via illumination apodization

Abstract

A video measurement system for measuring a test object comprising an imaging system having an imager having an imaging pupil, the imager arranged for viewing at least a portion of a silhouette of a test object by receiving light transmitted by the test object over a first angular extent, an illumination system having an optical axis and emitting an illumination distribution of light along an optical path, the illumination system comprising a lens system and a subassembly having an illumination source, a pupil aperture, and a first polarizer, the first polarizer having a diameter that is smaller than the pupil aperture, and a second polarizer overlapping the first polarizer, the second polarizer having a diameter that is larger than the first polarizer, the second polarizer configured to rotate independent of and relative to the subassembly.

1 . A video measurement system for measuring a test object comprising:
 an imaging system having an imager having an imaging pupil, the imager arranged for viewing at least a portion of a silhouette of a test object by receiving light transmitted by the test object over a first angular extent;   an illumination system having an optical axis and emitting an illumination distribution of light along an optical path, the illumination system comprising a lens system and a subassembly having an illumination source, a pupil aperture, and a first polarizer, the first polarizer having a diameter that is smaller than the pupil aperture; and   a second polarizer overlapping the first polarizer, the second polarizer having a diameter that is larger than the first polarizer, the second polarizer configured to rotate independent of and relative to the subassembly.   
     
     
         2 . The video measurement system of  claim 1 , wherein the first polarizer and second polarizer are linear polarizers. 
     
     
         3 . The video measurement system of  claim 1 , wherein the first polarizer is in a fixed position relative to the optical axis of the illumination system and is in approximately the same plane as the pupil aperture. 
     
     
         4 . The video measurement system of  claim 1 , wherein the imaging pupil is substantially aligned with the first polarizer. 
     
     
         5 . The video measurement system of  claim 1 , wherein the subassembly of the illumination system further comprises a light pipe having an entrance end and an exit end, and wherein the first polarizer is coupled to the exit end of the light pipe. 
     
     
         6 . The video measurement system of  claim 1 , wherein the subassembly further comprises a diffuser in substantially the same plane as the first polarizer and the pupil aperture. 
     
     
         7 . The video measurement system of  claim 6 , wherein the diffuser is a surface diffuser. 
     
     
         8 . The video measurement system of  claim 6 , wherein the diffuser is a volumetric diffuser. 
     
     
         9 . The video measurement system of  claim 1 , further comprising a quarter waveplate, wherein the second polarizer is positioned between the quarter waveplate and the first polarizer. 
     
     
         10 . The video measurement system of  claim 1 , wherein the second polarizer is a circular polarizer coupled to and rotationally aligned to a quarter waveplate. 
     
     
         11 . The video measurement system of  claim 1 , wherein the illumination system further comprises an output having a second angular extent in object space that is larger than the first angular extent received by the imager, and wherein the pupils of the illumination and imaging systems are in conjugate image planes. 
     
     
         12 . A method of measuring a test object comprising:
 (a) presenting a test object within a video measuring system, the video measuring system having an imaging system and an illumination system;   (b) emitting an illumination distribution of light along an optical path with the illumination system, the illumination system comprising a lens system and a subassembly having an illumination source, a pupil aperture, and a first polarizer, the first polarizer having a diameter that is smaller than the pupil aperture;   (c) capturing at least a portion of a silhouette of a test object with an imager having an imaging pupil by receiving light transmitted by the test object; and   (d) rotating a second polarizer independent of and relative to the subassembly to change a ratio of illumination passing through both the first and second polarizer, wherein the second polarizer (i) has a diameter that is larger than the first polarizer and (ii) overlaps the first polarizer.   
     
     
         13 . The method of  claim 12 , further comprising the step of measuring the silhouette of the test object. 
     
     
         14 . The method of  claim 12 , further comprising the step of applying a quarter wave plate to the second polarizer, wherein the second polarizer is positioned between the first polarizer and the quarter wave plate. 
     
     
         15 . The method of  claim 12 , further comprising the step of diffusing the light emitted by the illumination system. 
     
     
         16 . A video measurement system for measuring a test object comprising:
 an imaging system comprising an imager having an imaging pupil, the imager arranged for viewing at least a portion of a silhouette of a test object by receiving light transmitted by the test object over a first angular extent;   an illumination system, the illumination system comprising an illumination source emitting an illumination distribution of light along an optical path, a lens system, a second angular extent that is larger than the first angular extent received by the imager, and an output, wherein the pupils of the illumination and imaging systems are in conjugate image planes;   a subassembly within the illumination system, the subassembly comprising a pupil aperture and a first polarizer, the first polarizer having a diameter that is smaller than the pupil aperture; and   a second polarizer overlapping the first polarizer, the second polarizer having a diameter that is larger than the first polarizer, the second polarizer configured to rotate independent of and relative to the subassembly.   
     
     
         17 . The video measurement system of  claim 16 , wherein the subassembly further comprises a diffuser. 
     
     
         18 . The video measurement system of  claim 17 , wherein the diffuser is a surface diffuser. 
     
     
         19 . The video measurement system of  claim 17 , wherein the diffuser is a volumetric diffuser. 
     
     
         20 . The video measurement system of  claim 17 , wherein the diffuser is in substantially the same plane as the first polarizer and the pupil aperture 
     
     
         21 . The video measurement system of  claim 16 , further comprising a quarter waveplate, wherein the second polarizer is positioned between the quarter waveplate and the first polarizer. 
     
     
         22 . The video measurement system of  claim 16 , further comprising a circular polarizer having the second polarizer coupled to and rotationally aligned to a quarter waveplate.